If you want yours, well...yes, you can, via the Darkspore Creature Converter, so long as you haven't uninstalled Darkspore (uninstalling Darkspore deletes the folder containing the PNGs).

If you want the default heroes, here are the PNGs for all the creatures in Darkspore, ripped straight from the game files. Just run the ones you want through the Darkspore Creature Converter and you're good to go! ^-^